The prevalence of cardiovascular disease will rise as life expectancy of older Americans continues to increase, with persons aged >75 years representing the fastest growing segment of the US population. Poor physical fitness is a major contributor to poor cardiopulmonary function that is primarily caused by a sedentary lifestyle. Increasing physical activity remains a priority for improving cardiovascular health, especially in older adults, who are at the greatest risk of chronic health conditions. While it is generally recognized that physical activity benefits adults regardless of age, sex, race/ethnicity, or health status, the cardiopulmonary responsiveness varies greatly. Approximately 40% of people do not achieve a clinically meaningful benefit despite excellent adherence. On the other hand, 30% of people with poor adherence respond better than expected. Based on our previous observation that both common and rare nonsynonymous mitochondrial DNA (mtDNA) variants are associated with physical activity energy expenditure and cardiopulmonary outcomes, we postulated that these variants are likely to identify cardiopulmonary responsiveness to chronic physical activity. These data formed our central hypothesis that mtDNA sequence variation explains a portion of the heterogeneity in cardiopulmonary responsiveness to chronic physical activity. We have a unique opportunity to test our central hypothesis efficiently and cost-effectively by sequencing the entire 16.5kb of mtDNA in stored samples of participants in the Lifestyle Interventions and Independence for Elders Study (The LIFE study). The LIFE study is a definitive Phase 3 multicenter single-masked Randomized Controlled Trial that evaluates a physical activity program vs. a successful aging health education program. The average follow-up duration of the study is approximately 2.7 yrs, and the participants are 1,592 community-dwelling sedentary persons aged 70-89 yrs with stored genetic material. The completed LIFE Pilot study-a cohort of 396 participants randomized to the same interventions for 12 months-will be used to replicate significant associations. Our hypotheses address the effect of common and rare mtDNA variants on responsiveness to the following cardiopulmonary measures that are being collected as part of the trial: 1) walking speed, 2) blood pressure, and 3) pulmonary capacity. We will integrate clinical, behavioral, and genetic data in models to predict the heterogeneity in cardiopulmonary responsiveness to physical activity. By identifying genetic modifiers, this research will provide a starting point to build a personalized medicine framework to better improve cardiovascular health with physical activity. Identifying these genetic factors may also provide novel insights into the molecular pathways that regulate the cardiovascular adaptation to chronic physical activity. This approach could have a large impact in moving the field toward the NIH's goals of personalizing behavioral interventions for a rapidly aging America.